From mboxrd@z Thu Jan 1 00:00:00 1970 From: Mark Lord Subject: Re: SATA RAID5 speed drop of 100 MB/s Date: Sun, 24 Jun 2007 19:46:58 -0400 Message-ID: <467F0272.9070903@rtr.ca> References: <4679B2DE.9090903@garzik.org> <20070622214859.GC6970@alinoe.com> <467CC5C5.6040201@garzik.org> <20070623125316.GB26672@alinoe.com> <467DA1F5.2060306@garzik.org> <467E5C5E.6000706@msgid.tls.msk.ru> <20070624125957.GA28067@gallifrey> <467E9356.1030200@msgid.tls.msk.ru> <20070624220723.GA21724@alinoe.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: Received: from rtr.ca ([64.26.128.89]:3247 "EHLO mail.rtr.ca" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751418AbXFXXrB (ORCPT ); Sun, 24 Jun 2007 19:47:01 -0400 In-Reply-To: <20070624220723.GA21724@alinoe.com> Sender: linux-ide-owner@vger.kernel.org List-Id: linux-ide@vger.kernel.org To: Carlo Wood , Justin Piszcz , Michael Tokarev , "Dr. David Alan Gilbert" , Jeff Garzik , Tejun Heo , Manoj Kasichainula , linux-kernel@vger.kernel.org, IDE/ATA development list Carlo Wood wrote: > > The following can be observed: > > 1) There is hardly any difference between the two schedulers (noop > is a little faster for the bonny test). > 2) An NCQ depth of 1 is WAY faster on RAID5 (bonnie; around 125 MB/s), > the NCQ depth of 2 is by far the slowest for the RAID5 (bonnie; > around 40 MB/s). NCQ depths of 3 and higher show no difference, > but are also slow (bonnie; around 75 MB/s). > 3) There is no significant influence of the NCQ depth for non-RAID, > either the /dev/sda (hdparm -t) or /dev/sdd disk (hdparm -t and > bonnie). > 4) With a NCQ depth > 1, the hdparm -t measurement of /dev/md7 is > VERY unstable. Sometimes it gives the maximum (around 150 MB/s), > and sometimes as low as 30 MB/s, seemingly independent of the > NCQ depth. Note that those measurement were done on an otherwise > unloaded machine in single user mode; and the measurements were > all done one after an other. The strong fluctuation of the hdparm > results for the RAID device (while the underlaying devices do not > show this behaviour) are unexplainable. > >>>From the above I conclude that something must be wrong with the > software RAID implementation - and not just with the harddisks, imho. > At least, that's what it looks like to me. I am not an expert though ;) I'm late tuning in here, but: (1) hdparm issues only a single read at a time, so NCQ won't help it. (2) WD Raptor drives automatically turn off "read-ahead" when using NCQ, which totally kills any throughput measurements. They do this to speed up random access seeks; dunno if it pays off or not. Under Windows, the disk drivers don't use NCQ when performing large I/O operations, which avoids the performance loss. (3) Other drives from other brands may have similar issues, but I have not run into it on them yet. Cheers